1. Field of the Invention
The present invention relates to the field of optical devices, and particularly to light modulators such as traveling-wave modulators, phase shifters, and switches.
2. Description of Related Art
Telecommunication companies seek to increase the amount of information throughput with fatter pipes and at higher speed to meet the demand from the industrial, business, and consumer markets. This in turn requires a light transmitting system to enlarge transmission and receiving capacity drastically. At present, the light transmission speed of 10 Gb/second has already been reduced to practice for commercial use, with the next hurdle set at 40 Gb/second.
Several testings are underway to find a suitable material for use as an optical waveguide in a traveling-wave light modulator that is capable of operating in broad band at high frequency, such material includes lithium niobate (LiNbO), lithium tantalite, potassium lithium niobate, potassium titanyl phosphate or gallium-arsenide. Lithium niobate and lithium tantalite are excellent ferroelectric materials, with large electro-optical coefficients, which can control a light phase proportional to an electrical field strength within an optical waveguide generated by an electrical signal applied to electrical electrodes.
Factors which effect the modulation of a traveling-wave light modulator include velocity mismatch, impedance mismatch, dispersion, electrode power loss, and the electrical field generation. Among them, velocity mismatch, impedance mismatch and dispersion are principally determined by the structure of the traveling-wave light modulator, which can be achieved with careful electrode design. However, the electrical field strength is determined by the applied electrical signal amplitude, the electrode power loss and the electrode structure, and the excitation mode in the electrode structure. For 40 Gb/sec. modulation, one of the major challenges is to reduce the required driving voltage of the modulator, which is generally dictated by high electrode loss and the difficulties of generating high-voltage swing with semiconductors at this speed.
In electrical field generation and the phase modulation, at the input of one electrical waveguide or electrode, a high-speed electrical signal is applied and triggers an electromagnetic wave propagating along the waveguide. The field strength at a certain point along the waveguide is determined by the particular way in which the EM wave was excited for a given input voltage, and the propagation attenuation along the waveguide. The optical index of the optical waveguide is changed linearly by the applied electrical field, and the overall phase change of the optical signal is an integration of all the incremental phase changes along the waveguide and is proportional to the product of the driving voltage and the modulation length. Due to bandwidth considerations, the effective modulation length cannot be increased beyond a limit and hence a driving voltage above a threshold is required to achieve a required optical modulation. For high-speed communications systems transmitting at 10 Gb/sec or higher, the electrode loss is significant and typically leads to a very high required driving voltage.
Given the high loss and the limited voltage swing, it adds more complexity and cost to realize a practical communication system using such a modulator, if not impossible. The under-driven modulator would lead to significant degradation of the modulated light signal and significantly limits its use to many communication systems. Therefore, a light modulator having a lower driving voltage is in demand.
Attempts have been made to reduce a driving voltage. One method has been a two stage electrode design which uses the first stage of the electrode to primarily achieve the maximum overlap of the electrical field and the optical field, and uses the second stage to achieve the phase velocity match the between the electrical and the optical signals. However, it is difficult to realize due to phase matching required of the two stages of the electrode. Further, it just introduces one more freedom to alleviate the constraints for simultaneous phase and field matching. It does not provide an effective means to reduce the driving voltage.
A ridge structure is a modification of a conventional CPW (co-planar waveguide) design, by raising the center electrode conductor above the two grounding planes. It does provide the advantage of lowering the driving voltage. For example, see K. Noguchi et al, “Highly efficient 40-GHz bandwidth Ti: LiNbO optical modulator employing ridge structure”, IEEE Photonics Technology Letters, Vol. 5, No. 1, January 1993. However, it is difficult to realize due to the additional processes and the additional optical signal losses incurred by fabricating the ridge. Moreover, the reduction of the driving voltage is very limited, which is about 20% typically.
A conventional broadband optical communication uses a Mach-Zehnder interferometer to modulate laser signals in a transmitter. An electric field applied to an optical waveguide changes its index of refraction. A signal strip and ground plane (a zero voltage), form an electrical waveguide (EWG), where the induced electric field creates a change in the refractive index of the inlayed optical waveguide (OWG). The index of the material, for example, LiNbO3 or GaAs, depends on the amplitude and direction of the applied electric field.
Lithium-Niobate Mach-Zehnder modulators require a large voltage and length to provide a π phase shift through an active length L. The voltage level required is too large relative to amount provided by ultra-fast electronic transistors. The length of the modulator is limited by the synchronism of the electric and optical propagating waves. For this reason, the length cannot be increased without a regenerative amplification of the signal or a multistage system that requires precise synchronization.
Accordingly, it is desirable to have phase shifters, modulators, and methods that decrease a Vπ value or shortening of an active length.